1. Field of the Invention
The present invention relates generally to the field of semiconductors, particularly to manufacturing methods for fabricating semiconductor devices, and more particularly to the Back-End-Of-Line (BEOL) semiconductor manufacturing process.
2. Description of the Prior Art
The semiconductor manufacturing process, when likened to an assembly line, includes two major components, namely the Front-End-of-Line (FEOL) which includes the multilayer process of the actual forming of semiconductor devices (transistors, etc.) on a semiconductor substrate, and the Back-End-Of-Line (BEOL) which includes the metallization after the semiconductor devices have been formed. Like all electronic devices, semiconductor devices in a microchip such as an integrated circuit (IC) need to be electronically connected through wiring. In an integrated circuit, such wiring is done through multilayer metallization on top of the multilayered semiconductor devices formed on the semiconductor substrate. The complexity of this wiring becomes immediately appreciable once one realizes that there are usually hundreds of millions or more semiconductor devices (transistors in particular) formed on a single IC, and all these semiconductor devices need to be properly connected. This is accomplished by multilayer metallization, with each metallization layer designated as Metal 1, Metal 2, so on, where Metal 1 is the metallization layer closest to the underlying semiconductor devices to provide local connections among neighboring devices, and other metallization layers provide increasingly global connections from Metal 2 to the top metallization layer. Each metallization layer consists of a grid of metal lines sandwiched between dielectric layers for electrical integrity. Modern semiconductor manufacturing process can involve six or more metallization layers.
Although in the early years of semiconductor industry BEOL was generally less important than FEOL, the recent advancements have changed that equation. Microchip interconnect technology has become a critical challenge for future IC advancements due to the increasing difficulties to reduce signal propagation delay or interference caused by the increasingly dense interconnects. The problem is particularly acute considering that while an increase of metallization density means longer signal delays caused by the interconnects, a corresponding increase of transistor density means shorter signal traveling time between local semiconductor devices, making metallization increasingly a bottleneck in enhancing IC performance.
Enhancements in integrated circuit (IC) density and performance as predicted by Moore's Law have fueled the semiconductor industry and resultant Information Revolution for over 40 years. This has resulted in a great number of developments in BEOL process of semiconductor manufacture. Among these changes, the most dramatic type is probably changing from using one type of metal for metallization to using a different metal, as exemplified by the aluminum (Al) to copper (Cu) transition. The fabrication of deep submicron Ultra-Large Scale Integrated (ULSI) circuits requires long interconnects having small contacts and small cross-sections. In the past generation of semiconductor manufacturing process technology (industry generally considers 0.13 μm a boundary), aluminum (Al) and Al alloys have been used as conventional chip wiring materials while tungsten (W) has been used as contact plug between metal layers. The newer generation of the semiconductor manufacturing process technology has made it necessary to replace the Al technology with a technology based on a different metal. The introduction of copper (Cu) metallization served as an enabler for aggressive interconnects scaling due to its lower resistivity (e.g., the effective resistivity of TaCu in a 1 μm line is ˜2.2 μΩ-cm) as compared with traditional Al metallization (e.g., the effective resistivity of Ti/Al(Cu)/Ti in a 1 μm line is ˜3.3 μΩ-cm) as well as improved reliability (such as less electromigration) and reduced steps of fabrication.
The transition from Al technology to Cu technology has been dramatic. After years of development activities, Cu technology first started to be implemented in some advanced ICs such as microprocessors, but since then has quickly swept nearly the entire semiconductor manufacturing industry. In addition to the technological mandate for Cu technology as discussed above, the phenomenon of the rapid Cu dominance also has to do with the dynamics and the economy of semiconductor manufacturing industry. For example, because it is generally less expensive to maintain a single type of manufacturing line than to maintain two different types of manufacturing lines with one line implementing Al technology and the other Cu technology, semiconductor manufacturers generally choose to complete the Al to Cu transition in a sweeping manner, even though there are still many product designs that could be adequately manufactured using the conventional Al technology. Currently, most of chipmakers have simply moved from Al interconnect to Cu interconnect fabrication (BEOL backend of line process) by following semiconductor roadmap when they are ready for 0.13 μm technology or below. With Cu technology becoming more mature, continue to invest in Al process is no longer desirable or even practical since it will add tremendous overhead to operations and facility managements. In addition, Al process can only be utilized up to 0.15 μm and clearly has a limited service life span, making further investments by chipmakers in the desolating technology more difficult.
On the other hand, the semiconductor manufacturing industry has gone a step ahead of the product/application industry in the Al-to-Cu transition. Most chipmakers or fabrication service providers serve a broad range of customers having a variety of products, many of which have been designed based on Al technology. From a standpoint of the product design alone, many of these Al designs need not be fabricated using 0.13 μm (or below) technology and thus need not be designed based on Cu process. This led to an undesirable situation where a company that has a product design based on Al process will have to either redesign the product for Cu process or find a chipmaker who still maintains a production line based on Al technology, which is becoming increasingly more difficult or less economical.